We had a customer recently who changed their Outlook Anywhere Authentication from NTLM to Basic.
Shortly after doing this every single user started getting Basic Authentication prompts including internal users who were not connecting via Outlook Anywhere.
We checked the Authentication on the RPC directory in IIS and noticed that Windows Authentication was disabled.
We decided to enable Windows Authentication and this sorted the issue for around 10 minutes and then users started prompting again. We checked the authentication on the RPC directory again and it had disabled itself again.
We then looked at the Outlook anywhere settings in PowerShell via running Get-OutlookAnywhere
We then looked the identity and could see it was referencing the RPC directory as you would of thought and then we looked at the IISAuthenticationMethod and noticed that it just said “Basic”, so this got the alarm bells ringing that this was the setting that was applying every 15 minutes to revert the IIS authentication.
So we ran the following command “Set-OutlookAnywhere -IISAuthenticationMethod Basic,NTLM“. This enabled windows Authentication on the RPC directory straight away and the setting never reverted J
So BEWARE that changing your Outlook Anywhere authentication setting to Basic removes the NTLM Authentication method from the RPC Directory!!!!